Conventionally, there have been used in the field of powder metallurgy compression molding, in which powder is packed in a mold and compressed in order to mold a product with powder as a raw material, a method of placing a compressed green compact in a high-temperature furnace to sinter the same, and the like. In recent years, a demand for high purity and high density of products has become great, and so there has been used a hot isostatic pressing method of vacuum-sealing powder in a can made of thin steel sheet or glass, placing the can in a furnace, which is filled with gases such as Ar, N2, or the like, and applying high temperature and high pressure thereto to achieve compression sintering.
In case of molding a product by the hot isostatic pressing method, however, there is a danger of deformation of the product or bursting of a can due to an increase in contraction ratio at the time of molding in the event that powder being a raw material is low in packing density.
Therefore, a high packing density is demanded of a powder material used in the hot isostatic pressing method.
Spheroidization of a powder material is known as measures of enhancing such packing density.
A spheroidized powder is enhanced in flowability to afford uniform packing. In particular, even powder of high cohesiveness is effective because of a rapid increase in packing density when it is prepared to make a spheroidized powder of a predetermined particle size.
Also, since an expensive powder material gives rise to a great demand for a near net shape, a spheroidized powder affording a high packing density is effective.
Also, a spheroidized powder material is needed in the field of plasma spraying with a view to an increased flowability in powder supply. Further, a spheroidized powder or a near spheroidized powder is demanded in use for catalyst and chemical industries.
As a method of preparing such spheroidized powder, for example, JP-A-2002-180112 presented by the applicant of the present application discloses a method of using plasma.
In a technique disclosed in JP-A-2002-180112, a mixed gas of Ar and H2 is used to generate high frequency induction thermal plasma, powder is passed through plasma having temperature of 3000 to 10000° C. to melt and spheroidized by surface tension, a reducing gas is used to remove oxygen or impurity substances of low melting point contained in the powder, the molten powder falls in a chamber while being cooled, and coagulates as it is to provide a spheroidized powder of high purity.
The method of spheroidizing a powder material with the use of plasma is advantageous and effective in terms of removal of impurities as described above.
However, thermal plasma generated in electromagnetic induction of high frequency electric power principally involves the skin effect, so that temperature distribution in plasma flame is not even. Actually, a high-temperature region is formed in an outer peripheral portion of plasma flame and a low-temperature region is formed in a central portion of plasma flame. The technique described in JP-A-2002-180112 is advantageous in that a central low-temperature region is made the most of to permit a powder supply nozzle to be inserted centrally of plasma flame to enable passing every supplied powder through the flame for processing. On the other hand, as a result of examination conducted by the inventors of the present application, however, the inventors were confronted with a problem that as powder supplied into plasma is increased in amount, spheroidization does not advance adequately.
This is because, while damage against the powder supply nozzle is less when a powder material is supplied from that neighborhood radially centrally of a high frequency coil, which is included in the low-temperature region, a powder material gushing out linearly from the powder supply nozzle falls without passage through the high-temperature region of plasma, thus plasma energy can not be used effectively for the powder treatment.
As an approach to this problem, a technique is proposed, in which a powder material is acted upon at an outlet of a nozzle by external gases in a manner like gas atomization and is diffused in a high-temperature region of plasma formed in a doughnut-shaped manner (Thermal Spray. Practical Solutions for Engineering Problems, C. C. Berndt (Ed.), Published by ASM International, Materials Park, Ohio-USA, 1996 P683-690).
Because of the ability to make effective use of thermal energy, the technique of supplying a powder material to a high-temperature region of plasma achieves an increase in efficiency of spheroidizing processing and so is effective in industrial production. However, the technique proposed in the above document by C. C. Berndt involves a problem of increased control factors because of the use of a third gas as a drive force for diffusion of powder, and a problem of the necessity of maintaining a mechanism, which serves to supply the third gas and is exposed to high temperature, in a controllable state, and the inventors of the present application have not been able to apply the technique to industrial production.
Also, the inventors of the present application have examined a method of supplying a powder material not to a center of a high frequency coil, that is, a low-temperature region of plasma but directly to a high-temperature region of plasma. However, powder particles connect together due to an increased supply of the powder material and melt as they are, to show a tendency that powder containing coarse particles results, thus making application to industrial production difficult.